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Basement perimeter drains, French Drains, or B-Dri/Beaver Basement type basement de-watering systems: this article series discusses types of interior perimeter drain or what some call a "french drain" for stopping foundation leaks or preventing wet basements and crawl spaces. We describe the basic types of exterior and interior foundation drainage and de-watering systems for homes.

A perimeter drain, as we explain in this article, is an indoor drain cut into the floor around the perimeter of a basement or crawl space to intercept and remove water from the building interior. We illustrate perimeter drains and we comment on their effectiveness and installation details below.

Interior foundation perimeter drains typically intercept water leaking through a foundation wall or around the building wall/floor slab intersection. The intercepted water is conducted to a sump pit and sump pump for disposal, or it may be conducted to a location from which it can drain by gravity.

Some people call an interior perimeter drain used for basement or crawl space de-watering a "French Drain". Whatever you want to call it, an interior perimeter drainage system cut into the basement or crawl space floor can effectively stop basement or crawl space flooding - or can it?

In our opinion an interior drainage system can work to keep water off of the floors, and properly installed it can also resist sending problematic high moisture levels into the building as well.

But before installing a costly basement interior drainage system, it makes sense to check and repair obvious outdoor water entry sources that send water through the building foundation walls. That is especially true if the outdoor water entry causes are trivial and inexpensive to cure.

Carson Dunlop Associates' sketch (left) shows three different approaches to installing an interior drainage system to stop basement flooding.

What are the Definitions of a French Drain, Footing Drain, & Perimeter Drain?

Well plenty of people do call interior foundation drains or perimeter drains a "french drain". We don't.

A "French Drain" is an outdoor buried drain line constructed to carry water away from the building, typically to a drywell or catch basin. Our sketch shows how we remove water from roof runoff that pours down a downspout.

So what is the difference between a french drain, a footing drain, and a perimeter drain. A French drain is shown above, and a footing drain is shown in our two sketches below.

A footing drain, that is an exterior foundation drainage system placed outside the foundation wall near the wall footing, at the level we show, covered with gravel, and if the footing drain going to do anything, it is piped to daylight or to a catch basin that is in turn pumped to daylight or to a storm drain.

Are Indoor Perimeter Drains a Foolproof Cure for Wet Basements ?

Skip the exterior excavation and waterproofing - an interior perimeter drainage system can work just as well, according to Scott Anderson, The Journal of Light Construction, December 2005 [1].

Watch out: our OPINION is that while Mr. Anderson offers good design details for a basement interior perimeter trench and drain system, and though he reports success with this system, we think it's mostly appropriate for an un-finished basement in a home where high moisture from water running down walls is not a concern.

If a basement de-watering design is successful only at keeping water off of the floors, that is if it does not also keep excessive humidity from passing into the basement through walls and floors, the building is still at great risk of high indoor moisture levels, condensation problems, indoor mold, and related troubles.

The basement WaterGuard™ interior basement drain system described in this article shows water running down the basement wall and into the drain system.

Also we notice that the perimeter drain in the example installation is routed to an indoor sump pump that sends water up, out through the basement wall, into a short plastic drain pipe extending about 18" away from the foundation, and thence into an open plastic trench on the ground surface.

If that basement water disposal system is blocked by snow and ice, the sump system will simply stop working and the basement will flood, possibly worse than ever as we have also drilled openings to improve flow of water from the foundation wall into the interior perimeter drain.

Wonderful. And nothing is stopping evaporation of water from the wall surface into the basement area where it causes indoor humidity to skyrocket.

Below we show two implementations of the type of basement perimeter drain de-watering system sketched above. At below left you can see that a trench was cut into the floor and something was buried there, along the foundation wall: typically perforated pipe in a gravel-filled trench that slopes around the floor to end at either a sump pump pit or if you are lucky, a spot where you can drain water out of the building to daylight using just gravity.

The installer left a 1/4-inch gap to encourage water that runs down the wall to flow into the drainage trench. This system will usually keep water off of the basement floor quite successfully but it does little to keep moisture out of the area as water evaporates from the wall surface as well as flowing into the trench.

A second common basement de-watering system shown at above-right is also an indoor foundation perimeter drain system like the one we described above. But this installation includes heavy plastic that extends a few inches up the wall, perhaps to encourage water to run behind the plastic and into the gravel trench. This approach also covers up an ugly wet water stained row of concrete blocks at the bottom of the foundation wall. Early B-Dry™ type basement drainage systems used this approach.

At contemporary B-Dry™ and similar installations that we have inspected the installers have shifted to running this heavy plastic barrier up the entire face of the foundation wall, right up to the sill plate. We like that improvement as it should dramatically reduce moisture movement into the room itself as water runs down the wall and into the drainage trench.

In that system one is, of course, unable to see the foundation wall to gain any understanding of just when and where water is entering the structure.

Trenchless Basement Water Interception Systems

A "Beaver Basement™" de-watering system is a simpler basement perimeter catch-and-drain system.

In the installation shown here for a Duluth Minnesota home, the basement dry-out approach uses what looks like a heating baseboard to intercept water passing down or through the foundation wall.

Water runs inside a channel, around the room, and into a drain or sump system. This system, omits the step of cutting open the basement floor to install gravel and drainage piping to the sump.

Depending on the source and quantity of water leaking into the structure, this less costly installation might work.

This article series includes articles reprinted/adapted/excerpted with permission from Solar Age Magazine - editor Steven Bliss.

Our home page for diagnosing and curing foundation leaks and wet basements or crawl spaces is WATER ENTRY in BUILDINGS. If your building has been flooded, see FLOOD DAMAGE ASSESSMENT, SAFETY & CLEANUP. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.

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